StealthChat is an industrial strength chat and phone call app for professionals that value real privacy and security. StealthChat uses advanced end to end encryption to ensure that only the sender and recipient have access to the content of the message or phone call.
XO Hosted PBX - Hosted PBX is a scalable UC solution that uses cloud technology to integrate HD voice & data services, mobility, along with advanced calling
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Element.io - Secure messaging app with strong end-to-end encryption, advanced group chat privacy settings, secure video calls for teams, encrypted communication using Matrix open network. Riot.im is now Element.
Sipdroid - Free and open-source SIP/VoIP Client
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.